Add a long running WordCount example

As far as I know, flink doesn't have a long running WordCount example for users to start with or doing some simple tests.  

The closest one is SocketWindowWordCount. But it requires setting up a server (nc -l ), which is not hard, but still tedious for simple use cases. And it requires human input for the job to actually run.

I propose to add or modify current WordCount example to have a SourceFunction that randomly generates input data based on a set of sentences, so the WordCount job can run forever. The generation ratio will be configurable.

This will be the easiest way to start a long running flink job and can be useful for new users to start using flink quickly, or for developers to test flink easily.